Problem: Luis is 3 times as old as Daniel and is also 20 years older than Daniel. How old is Luis?
Answer: We can use the given information to write down two equations that describe the ages of Luis and Daniel. Let Luis's current age be $l$ and Daniel's current age be $d$ $l = 3d$ $l = d + 20$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $l$ is to solve the second equation for $d$ and substitute that value into the first equation. Solving our second equation for $d$ , we get: $d = l - 20$ . Substituting this into our first equation, we get the equation: $l = 3$ $(l - 20)$ which combines the information about $l$ from both of our original equations. Simplifying the right side of this equation, we get: $l = 3l - 60$ Solving for $l$ , we get: $2 l = 60$ $l = 30$.